Interesting paper discussed on TWEVO today: Some researchers figured out four immune system genes that had different allele frequencies in English and Danish populations around 1350 AD. One allele was present in about 40% of the population before the big Black Death pandemic, and then about 70% afterwards! This is yet another real life example of Natural Selection: People without that allele tended to die before reproduction. And yes, this was the gene allele responsible for plague and HIV immunity. But interestingly, it's not really specific to Yersinia pestis or HIV, since one is a bacteria and one a retrovirus. Instead this is a gene for an enzyme that snips fragments off a pathogen protein, so they can be presented to CD8 lymphocytes for antibody production. And guess what? It seems to work for SARS-COV2 as well! Which could help explain why some people seem to never get Covid-19 and never develop antibodies.

It actually "supercharges" the immune system, but in doing that may increase susceptibility to autoimmune problems like Crohn's Disease. Of course, evolution doesn't care what goes wrong when you get beyond reproduction age!